Electronic systems applied to aerospace are susceptible to single-event effects (SEEs) and fail, and the influence of SEEs on electronic devices in aerospace equipment is increasing as the feature sizes of integrated circuits continue to reduce, so the SEEs have become a main failure mode in large-scale integrated circuits for aerospace.
As one of the SEEs, SET usually refers to a phenomenon that circuit nodes have instant current and voltage changes as a semiconductor device is bombarded by spatial single particles, the energy of the particles is deposited to cause collisional ionization of the particles, and the ionized charges are collected and transported under the action of a concentration gradient and an electric field.
As signals distributed most widely in synchronous digital systems and having highest frequencies, clock signals play an important role in integrated circuits. A clock distribution network (CDN), as a carrier of the clock signals, consists of multiple clock nodes. The clock nodes will produce soft errors after being bombarded by radiation particles, which, if serious, will result in failure of a circuit (even the whole chip). Therefore, the importance of the CDN is naturally self-evident. At present, the soft error ratio (SER) caused by upset of clock nodes is seldom studied in the industry, and CDN reinforcement methods are rarely seen in the literature.
In order to achieve the objectives of low power consumption and low skew, people have been constantly researching and exploring the structure of the CDN. Currently common CDN structures include tree clock networks (balance trees, H trees, X trees, etc.), mesh clock networks, fishbone clock networks, and hybrid clock networks. In addition, novel clock distribution networks such as resonant clock distribution networks have emerged. No matter in which clock distribution network with a topological structure, clock buffers/inverters are essential parts. As a basis of clock distribution, the clock buffers play a leading role in providing pure and accurate clock signals: they provide more flexibility to designers, allowing designers to align clock edges or move clocks forward or backward, so as to enlarge data valid windows; and at the same time, they can compensate for line length delays and provide unique chip timing to help engineers to design best circuits.
The influence of single-event effects induced by heavy ions, protons, neutrons and the like on the clock networks is mainly embodied in two special modes of circuit failure: radiation-induced clock race (also called clock glitches) and radiation-induced clock jitter. The radiation-induced clock race indicates that the collected charges cause the clock to hop to an error state so as to introduce a new clock edge, and this phenomenon will lead to error sampling of data in an edge-sensitive circuit. The radiation-induced clock jitter indicates that, when the charge collection caused by radiation particles approaches the clock edge, the clock edge deviates from its expected hopping time, causing an increase in clock jitter. The research results of N. Seifert et al. show that in an unreinforced, pulse-latch-based clock distribution network design, the Clock SER accounts for 50% of the entire chip-level SER; and in a trigger-based design, the SER caused by the radiation-induced clock race accounts for the vast majority of all clock path SERs (the SER caused by the radiation-induced clock jitter accounts for 2% of the total clock path SER).
The resistance of the clock distribution network to the single-event effects can be directly characterized by the number of transient pulses captured on each leaf node of the CDN, the width of the transient pulses and the like, and can also be indirectly characterized by the number of error sampling of a timing unit caused by transient pulses on clock signals in the design.
A single event upset driving circuit for clock leaf nodes based on an improved Muller C-element was proposed in A Robust Single Event Upset Hardened Clock Distribution Network (Oct. 12 to 16, 2008, P 121-124) published by A. Mallajosyula and P. Zarkesh-Ha in the IEEE International Integrated Reliability Workshop Final Report. This technique filters out single-event transient pulses propagated on the clock path by introducing a delay unit in a driving unit for clock leaf nodes and using a time-redundant reinforcement method. This will produce an additional delay, and at the same time, the width of the single-event transient pulses that can be filtered out by the driving circuit completely depends on the delay unit introduced. In addition, this technique can only be used for the reinforcement of the leaf node driving unit in the clock distribution network.